Agriculture Reference
In-Depth Information
1991). Some species form microsclerotia in
culture, which are irregularly shaped dark-brown
aggregations of thick-walled, monilioid cells.
However, these are not common with
R. solani
AG-8.
Rhizoctonia solani
and
R. oryzae
are
multinucleate.
Rhizoctonia oryzae
branches at
30 º-50 º from the main hyphae and forms
abundant irregularly shaped orange-pink or
salmon- to brown-colored sclerotia in culture,
1-3 mm in diameter. Both species can survive
in root pieces as rounded, thick-walled monilioid
cells.
Rhizoctonia
is diffi cult to quantify because of
low population densities in soil. Rhizoctonia
propagules can be extracted by sieving of organic
matter, plating of soil pellets, and elutriation
or baiting with various plant materials (Singleton
et al., 1992). A semiquantitative method using
wood toothpicks as bait has been particularly
useful (Paulitz and Schroeder 2005) and quantita-
tive DNA-based methods using real-time poly-
merase chain reaction (PCR) have been developed
to identify and quantify several
Rhizoctonia
species from soil and plants (Okubara et al.,
2008).
US greatly reduced the impact of Rhizoctonia
bare patch by using very heavy direct-seed drills
that caused extensive and deep disruption of soil
in the seed row. However, such drills are now
seldom used because they require large tractors
and high amounts of energy. A lighter, paired-
row direct-seed drill confi guration also reduced
Rhizoctonia disease, possibly because of a more
open canopy and quicker soil warming between
pairs of rows and residue removal between paired
rows (Cook et al., 2000).
Burning or otherwise removing stubble from
no-till fi elds does not generally reduce disease
severity (Smiley et al., 1996a; T.C. Paulitz,
unpublished data) or the amount of
Rhizoctonia
inoculum (T.C. Paulitz, unpublished data), pos-
sibly because the pathogen mainly survives in the
root system. However, stubble or crop residue
does affect the soil temperature and moisture
during the spring, often resulting in cooler soils
which are more conducive for
Rhizoctonia
damage
to young seedlings.
Protective seed treatments with chemicals
such as tebuconazole, difenoconazole, thiram, and
fl udioxonil often result in better seedling health,
expressed as more tillers, roots, and greater plant
height, but in most cases grain yield is not statisti-
cally increased (Mazzola et al., 1996a; Paulitz and
Scott 2006). Effects of crop rotation have been
variable because of the wide host range of
Rhizoc-
tonia
on other rotation crops (MacNish and Neate
1996; Mazzola et al., 1996a; Cook et al., 2002a).
No genetic resistance to Rhizoctonia root rot has
been detected in adapted cultivars of wheat but
resistance appears to reside in wild relatives of
wheat that have not yet been exploited (Smith
et al., 2003a,b).
Disease management
The most effective cultural management practice
involves controlling the green bridge by killing
volunteer crop plants and weeds with a preplant
herbicide at least three weeks before planting
(Smiley et al., 1992). Keeping the fi eld fallow
without a living host also reduces the severity of
disease, but only if the fallow period is long
enough for inoculum levels of the fungus to be
reduced (Roget et al., 1987). Paulitz (unpublished
data) showed that chemical fallow over two con-
secutive years was not enough to reduce inoculum
of
R. solani
AG 2-1 or
R. oryzae
in a higher pre-
cipitation area of the Pacifi c Northwest, but could
reduce hyphal activity in low-rainfall areas by the
end of the fallow season, and that this effect could
be carried over to the following crop. Reduced
mechanical fallow was more effective than chemi-
cal fallow.
Increased disturbance in the seed row can also
reduce disease (Roget et al., 1996). Farmers in the
TAKE-ALL
Take-all is the most damaging root disease of
wheat worldwide and can cause severe grain yield
losses when consecutive cereal crops are grown
(Asher and Shipton 1981; Hornby et al., 1998).
Take-all is the most important limiting factor
for winter wheat production in Western Europe.
